Adapter or caddy is fine. Can get them on most shopping sites for cheap.

IIRC from my old office PC slinging days, a lot of those cases with 5-1/4 bays usually had slots for mounting screws that would allow you to mount a 3.5 drive flush to one sideusijg 2 screws. Then you get a 1-3/4" 6-32 screw stand off, thread it into the drive, and use that to mount it to the other side of the 5-1/4 bay.

Did that a lot to really old reused cases where there were a ton of 5-1/4 bays but only one 3.5 bay.